496 Fi*of. Sedgwick on the May Hill Sandstone, 



and taking up in 1832 the task where I had left it in 1831, I 

 crossed other nearly symmetrical undulations till I established 

 the great jMerioneth anticlinal. It is the key to the structure of 

 North Wales, whether we take sections north and south from the 

 apseof the anticlinal to the mountains of Fcstiniog, or make parallel 

 east and west sections from different parts of the anticlinal across 

 the Berwyn chain, and thence to the carboniferous or triassic 

 groups on the English frontier ; for we have a base of Llanberris 

 slates and Harlech grits whereon to construct the sections *. From, 

 this base, then, I constructed a section over Great Arenig to the 

 Bala limestone, and from that limestone over the Berwyn chain, 

 near the line of the Llanrhaidr road ; and a second section 

 further south so as to pass down the Llanwdden valley; and 

 they gave similar results. From the base hue, or Merioneth 

 anticlinal, to the Bala limestone is an ascending section of 

 enormous thickness. Above the limestone the section on the 

 first line is still ascending for some thousand feet, when the 

 eastern dip is reversed so as to throw the bed^ into a synclinal 

 form, on the eastern side of which, and just beyond the water- 

 shed of the chain, the Bala limestone is again brought out, 

 beyond which we have a descending section through inferior 

 slate and porphyries. The second (or Llawdden) line repeats the 

 synclinal, and brings out the Bala limestone far west of the 

 southern Berwyn watershed. There are other complicated phae- 

 nomena in this section which 1 cannot dwell on heref. 



In like manner, during 1832, 1 examined the northern part of 

 the Berwyn chain. Older Cambrian rocks are there seen on the 

 highest crest ; and then after a great flexure and perhaps /am//, 

 are overlapped by a great series of the upper Bala beds, which 

 give an ascending section with a northern dip as far as Corwen. 

 I afterwards determined the place of a great group composed of 

 sandstones, slates, and more rarely of conglomerates, which form 

 the base of the Denbigh flag, and, along with that flag, range 

 in a position discordant to the Bala rocks. This group I believed 

 to be immediately superior to the Cambrian slates of Corwen, 

 though the sections were broken and discontinuous. 



All the facts and inferences above stated were discussed, and 



* By Harlech grits I mean a remarkable stage of coarse siliceous sand- 

 stone, sometimes almost a conglomerate, which overlies the great Llanberris 

 and Nant Francon slate quarries. Its finest exhibition is in the Rhinog 

 Fawir chain between Festiniog and Dolgelly. In the Government Survey it 

 is called Barmouth sandstone ; and if I mistake not, includes a larger 

 group of strata than I include under the Harlech grits. As the typical 

 grits under Harlech Castle are a detached mass of the rock, I am not sure 

 that my name Harlech was well-chosen. 



t Such e. g. as the repetition of the Bala limestone by a fault, and the 

 entire inversion of the beds at the eastern end of the section. 



